The present invention is directed to lasers and, more particularly, the present invention is directed to an unstable laser resonator.
Unstable laser resonators typically include an excitation source, a gain or lasing medium and a pair of reflecting mirrors, all of which make up a laser cavity. The excitation source generates photons within the gain medium, some of which travel along the axis of the gain medium and are reflected back toward the gain medium by the opposing mirrors for subsequent amplification. In an unstable laser resonator, the diameter of the light is allowed to increase as it travels back and forth within the cavity. A portion of the light whose dimensions exceed a certain value is extracted to form the output beam. Many unstable laser resonator techniques employed require complex optical schemes and filter methods that are inefficient and cumbersome to align. One common and simple method is the confocal unstable laser resonator, which includes a combination of a concave high reflecting mirror positioned at one end of the laser cavity, and a meniscus lens with convex surfaces at the opposite end of the laser cavity. Although the light output from a confocal unstable laser resonator is collimated, it is difficult to achieve diffraction limited output, primarily due to the use of a reflective concave mirror.
While the typical unstable laser resonator arrangement may be suitable for certain applications, the inventor of the present application has improved upon the efficiency of the typical system. The present invention provides diffraction limited performance with the simplicity of earlier unstable laser resonator designs like the confocal unstable laser resonator. Moreover, the present invention may also be applied to low gain lasers where other unstable laser resonator designs are not easily applied.
Therefore, an object of the present invention is to provide an unstable laser resonator that does not need elaborate optical configurations or lossy intra-cavity apertures to reduce the number of modes that are allowed to oscillate within the laser cavity.
A further object of the present invention is to provide an unstable laser resonator with an increased energy output.
These and other objects are attained in the unstable laser resonator of the present invention. The laser resonator of the present invention is able to reduce the number of transverse modes which oscillate within the laser cavity, without using complicated optical schemes. The objects of the invention are attained by simultaneously achieving small fresnel number conditions associated with low order mode laser beams and large fresnel number conditions associated with high laser energy extraction, resulting in an output beam of greater efficiency and low divergence.
The present invention of a laser resonator includes a laser cavity, a gain medium disposed within the cavity, a first reflector and aperture positioned adjacent one end of the laser cavity, and a second reflector positioned adjacent an opposite end of the laser cavity. While the first reflector may be shaped in various ways, it is shown for illustrative purposes throughout the specification as being, for example, a plane surface, a convex surface, or a concave surface. The surface contour of the reflector will in large part be dependent upon the gain medium.
For a diffraction limited output beam from the laser cavity, the following formulas may be applied to establish the optical components and their separation within the laser cavity: ##EQU1##
L represents the diffraction limited rayliegh length of the cavity for a given beam waist radius .omega..sub.0 and .lambda. is the wavelength of the laser. R represents the radius of curvature of the high reflecting mirror and F.varies. the focal length property of the output mirror/lens element. In the simplest configuration of the present invention the output mirror/lens element can be a single optical element. This element may be a meniscus lens with one surface acting as a mirror and the other surface acting as a lens. The radius of curvatures R.sub.1 and R.sub.2 for this optical element are defined as follows: ##EQU2##
The second reflector, also referred to as the output reflector or mirror, is also not limited to one specific shape. For example, the output mirror may be any combination of concave, plano or convex surfaces so long as the light reflected back into the cavity and out of the cavity is collimated.
The purpose of the two opposing mirrors is to reflect the light within the gain medium until the light beam has reached an energy threshold condition to escape the output mirror. The function of the aperture adjacent to the first mirror is to prevent slightly higher order mode light from entering the cavity and being amplified through waveguiding affects from the walls of the gain medium. Diffraction limited performance may be attained without the use of the aperture, however. The present invention has the additional feature of collimating the output beam as it escapes the output mirror. The output mirror both reflects the light within the cavity and collimates the light as it escapes.
The present invention creates an output beam with diffraction limited light, which allows for smaller focal spots to be generated with simple lenses. The result is greater cutting and drilling efficiency realized due to increase in energy density at the focal point. As an example, the present invention has increased drilling and cutting speeds in aluminum, various ceramics, silicon, stainless steel and other metals by more than an order of magnitude.
The unstable laser resonator of the present invention may be used in numerous high gain laser devices, such as excimer lasers, Q-switched solid state lasers, CO.sub.2 TEA lasers, dye lasers, nitrogen lasers, copper vapor lasers and the like. Additionally, the feedback mechanism of the output mirror is such that low gain lasers could be made to operate at their diffraction limit by proper selection of curvature and reflectivity of the optics.